This invention relates to a molding method, more specifically it relates to a method of making sand molds having a high hardness particularly at and about pocket areas thereof (pocket areas being those generally located centrally of a sand mold and convexly shaped such as shown by numeral 1 in the accompanying drawing). In order to improve a hardness of casting surfaces particularly of pocket areas in addition to those areas of a sand mold which are located between a mold frame and a pattern and roots of which stand on a pattern plate (hereinafter called as the frame areas), it is knwon in Japanese preliminary patent publication Sho-No. 5557353 for example to have mold sand within a mold frame squeezed simultaneously or substantially simultaneously from both vertical sides thereof, viz., a side in which a pattern is bedded down (hereinafter called as the lower or pattern side) and also a side opposite to said lower or pattern side (hereinafter called as the upper or opposite side). In this way of squeezing, it is essential to hold a stroke for squeezing the mold sand from the lower side under strict control, so that joint end surfaces of the frame and the sand shall be brought to a same level. To wit, if the squeezing from the lower side is made until it reaches a pressure sufficient enough to squeeze completely the sand as a whole, it shall be extremely hard to bring the joint end surfaces of the mold frame and sand to a same level. On the other hand, the control or suppression of a squeezing stroke from the lower side as mentioned above, that is, to such extent that the mold frame and sand is assured of their joint bottom surfaces to be at a same level after squeezing, will afford only such pressure which is insufficient to squeeze the sand, especially pocket areas thereof sufficiently hard.
Besides the above problem, a squeezing operation of a sand mold from the lower side is accompanied with the following drawbacks. To wit, when a squeezing stroke from the lower side is made larger so as to increase a squeezing pressure, a gap is produced inevitably between circumferential sides of a pattern and inner circumferential walls of a mold frame, which allows mold sand to fall down thereto. A sealing means has to be provided for the pattern plate in this instance to compensate said gap, whereby the pattern plate becomes expensive. In this instance, an additional lower mold frame is often provided so as to compensate the difference of heights of end surfaces of the mold frame and sand.
In U.S. Pat. No. 3,807,483, there is disclosed that a pattern plate is slightly elevated vertically upward after having a cavity of a mold frame filled up with mold sand and having the sand molded therein pneumatically. In this disclosure, its molding is operated statically without any dynamic squeezing, and the vertical movement of the pattern plate is made so as to compensate stress of mold sand produced thereby within the mold frame cavity by having the sand escaped portionally from the cavity. This is not comparable to this invention which will be described hereinafter.